Effect of Trolox C on the oxygenation reaction of prostaglandin endoperoxide synthase with cis,cis-eicosa-11,14-dienoic acid

Trolox C, a water-soluble derivative of alpha-tocopherol, stimulates the oxygenation of cis,cis-eicosa-11,14-dienoic acid (AH) by prostaglandin endoperoxide synthase at lower concentrations and suppresses the stimulated reaction at higher concentrations. Surprisingly, Trolox C does not affect the stoichiometric ratio between the rate of formation of the oxygenation product 11-hydroxy-12-trans, 14-cis-eicosadienoic acid (AOH) and the rate of disappearance of molecular oxygen. The ratio of the two rates, d[AOH]/-d[O-2], remains constant at 2/1 for a series of Trolox C concentrations and in the absence of Trolox C. Results indicate that AH reacts preferentially with Compound I of the enzyme and that Trolox C does not compete for Compound I. Enzyme inactivation begins with formation of an unproductive Compound I-tyrosyl radical (Compound I-X .) which has the same number of oxidizing equivalents as the conventional peroxidase Compound I. The stimulating effect of low concentrations of Trolox C can be explained by reduction of the oxyferryl heme so that Compound I-X . is reduced to a Compound II-X . species, the Compound II analog of Compound I-X .. Thus heme bleaching is prevented. A further one-electron reduction by Trolox C of Compound II-X . reforms the native enzyme, which permits enzyme recycling. Large concentrations of Trolox C inhibit reformation of native enzyme, reducing the extent of stimulation.